Fracture Behavior of Droppers under Low Current Pulses and Stress Coupling Cyclic Fields

In traction power supply catenary systems of high-speed railways, integral droppers convey current for the use of the electric locomotive. Based on real electro-mechanical coupling conditions of high-speed rail catenary droppers, the microstructure and fracture morphology formation and changes of the copper-magnesium alloy wire (Cu-Mg0.4) resulting from the effect of current pulses of low density (10 A/mm2), short durability (0.25 s), and interval (5 min) were studied in this work, accompanied by low dynamic stress (30 MPa) and compression strain (0.3%) under long-term working cycles. The simulation and test results indicated the temperature increase in the droppers in the working state was slightly higher than room temperature. In these experiments, a large number of micropores were generated in the pre-rolled alloy wire droppers. Under the long-term action of low stress and amplitude pulse coupling fields, the droppers retained the fiber structure characteristics without obvious recrystallization; however, similar shear band structures appeared in the fiber structures, which were bent and deformed, and the micropore density increased. The micropores aggregated into microcracks and expanded, resulting in a gradual decrease in plasticity, and the fracture mode transformed from ductile trans-granular fractures to brittle inter-granular fractures, corresponding to deeper and shallower dimples, respectively.